Optical unit and associated method

ABSTRACT

The invention relates to an optical assembly with a laterally graded reflective multilayer whose reflecting surface reflects incident X-rays under low incidence angles to produce a two-dimensional optical effect. The reflecting surface comprises a single surface conformed along two curvatures corresponding to two different directions. The invention also relates to a manufacturing method of such an optical assembly. The method includes coating a substrate already having a curvature. The invention also relates to a device for generating and conditioning X-rays for applications for angle-dispersive X-ray reflectometry. The device includes the optical assembly connected to an X-ray source so that X-rays emitted by the source are conditioned along two dimensions so as to adapt the beam emitted by the source to the sample, with the X-rays having different angles of incidence on the sample under consideration.

This application claims priority to PCT/FR03/01896, filed Jun. 19, 2003which claims priority to FR0300623, filed Jan. 21, 2003 and toFR0207546, filed Jun. 19, 2002.

The present invention generally concerns the optical assemblies withlaterally graded reflective multilayer, for reflecting X-rays under lowincidence angles.

It is specified that <<low incidence angles>> refers to incidence angleslower than a value of about 10° (the angle of incidence being definedwith respect to the reflecting surface).

More precisely, the invention concerns an optical assembly with alaterally graded reflective multilayer having a reflective surface forreflecting incident X-rays under low incidence angles, while producing atwo-dimensional optical effect.

The invention also concerns a method for manufacturing such an opticalassembly.

And the invention also concerns the generation and the conditioning ofX-rays for applications of angle-dispersive X-ray reflectometry.

By <<two-dimensional optical effect>> is meant an optical effect usingtwo different directions of space.

It can for example be a focalization on one point (from a point source),or a collimation of a beam whose rays are not parallel in any directionof space (e.g. a divergent conical beam).

And for producing such a two-dimensional effect, it is possible tocombine two one-dimensional optical effects.

It is e.g. possible to focalize a divergent beam coming from a pointsource along a first direction (i.e. focalizing such a diverging beam ona line of focalization, and not on a single point), and also focalizethe beam along a second direction, orthogonal to the first direction, inorder to actually focalize the resulting beam onto a single image point.

As mentioned above, the invention finds an application in the generationand the conditioning of X-rays for applications of angle-dispersiveX-ray reflectometry.

Other applications (which are non limitative) of the invention concernthe generation of X-rays, analytical applications of X-rays such asdiffraction, crystal diffraction, protein crystallography, textureanalysis, thin film diffraction, stresses measurement, reflectometry,X-ray fluorescence.

It is specified that the definition of <<laterally graded>> shall beprovided in this text.

Optical assemblies such as mentioned above are already known.

As an example U.S. Pat. No. 6,041,099 discloses multilayer opticalassemblies of the so-called Montel mirrors type—those being useable formodifying the optical characteristics of incident X-rays by creating atwo-dimensional optical effect.

This type of optics is a variant of the so-called Kirkpatrick-Baeztraditional optical scheme, which consists in aligning two mirrors whichare not bound, the mirrors being curved along two orthogonal directions,in order to create a two-dimensional optical effect.

According to an evolution of this configuration, the optics disclosed inU.S. Pat. No. 6,041,099 are associated in a side-by-side configuration(“side-by-side Kirkpatrick-Baez device”) and present a multilayercoating.

FIG. 1 a represents such an optical assembly 33, which comprises twomirrors 331, 332 associated side by side, the surfaces of these twomirrors presenting curvatures which are centered on two axes which areorthogonal one relative to the other.

It is specified that in this text the figures which concern the state ofthe art are referenced with a <<a>> indicia.

A limitation of these KB side-by-side optical assemblies (the acronym KBbeing used in reference to the term Kirkpatrick-Baez) precisely derivesfrom the fact that they are made of two distinct elements put side byside (two elementary mirrors having each a surface which is able toproduce a one-dimensional optical effect, these two optical effectsbeing superimposed in order to produce the desired two-dimensionaloptical effect).

It is indeed necessary to assemble these elementary mirrors with greatprecision, this corresponding to a delicate operation.

Moreover, in such optical assemblies the incident rays undergo tworeflections in order to produce the two one-dimensional opticaleffects—a reflection on each elementary mirror—which generates losses inintensity.

A goal of the invention is to give access to optical assemblies such asmentioned in the introduction of this text, and which would not beassociated to the limitations and drawbacks mentioned above.

Moreover, an aspect of the invention concerns the use of such opticalassemblies for applications of angle-dispersive X-ray reflectometry.

In such application, an incident X-ray beam is conditioned on a sampleto be analyzed in such a way that the incident X-rays have a range ofincidence angles θ on the sample which is considered (at the level ofthe image area) of the order of a few degrees.

The analysis of the intensity of the reflected X-rays as a function ofthe incidence angle θ allows determining characteristics such as thethickness, the structure, the density or the interfacial roughness of athin film of material present on the sample.

R(θ) measurements are thus carried out, where R is the measuredreflectivity and θ the angle of incidence of X-rays arriving on thesample.

Such an application concerns among others the analysis of thin films forthe microelectronics industry.

The technique of X-ray reflectometry is indeed then particularlyefficient for the analysis of very thin films (typically lower than 50nm), compared to so-called optical techniques such as e.g. ellipsometry(this technique being widespread in the semiconductor industry for thecontrol of thickness and structure of dielectric materials).

It is known to carry out measurements of X-ray reflectometry by usingdifferent types of equipment, and according to different methods.

According to a first type of known method, the dispersion of theincidences of the rays of the beam coming onto the sample is obtained bymoving mobile elements of the measurement device.

According to a first variant of this first type of method, R(θ)measurements are carried out by using an X-ray source and a planarmonochromator, the angle dispersion being obtained by having the samplepivoting around an axis which is orthogonal to the surface of the sampleand to the direction of the propagation of the X-rays.

An example of such a known configuration is represented in FIG. 2 a.

This figure shows an X-ray source S whose flux of X-rays is directedonto a monochromator M.

A sample E1 is carried by a sample carrier E2.

The sample E1 has on its surface a thin film E10 for whichcharacterization by reflectometry is sought.

The rays issued from the reflection on the monochromator are directedonto the sample. And after their reflection on the sample, an X-raydetector D shall receive the reflected rays and allow their analysis.

The arrow F shows the controlled displacements of the sample carrier andits sample.

In this known configuration, R(θ) measurements thus require the controlof the displacement of mechanical elements of the device.

This naturally has a consequence on the duration of the measurementoperations, as such displacements requiring great accuracy also requiretime.

According to a second variant of this first type of method, it is alsoknown to carry out R(θ) measurements by keeping the sample unmoved, butby controlling the displacement of the X-ray source and of the detectorwhich shall receive the rays after their reflection on the sample, themovements of the source and of the detector being controlled so as to besymmetrical with respect to the sample.

It shall be understood that in such case again, the displacements whichare carried out significantly impact the duration of the acquisition ofthe measurements.

And these known techniques of carrying out R(θ) measurements are thusassociated to relatively long operating times, which is in itself alimitation (e.g. for applications such as thin film analysis for themicroelectronics industry).

According to a second known type of X-ray reflectometry methods, theincidence dispersion of the rays of the beam arriving onto the sample isobtained through an optical assembly which is able to produce aone-dimensional or a two-dimensional optical effect.

This second type of method is known as angle-dispersive X-rayreflectometry.

The principle of such method is shown in FIG. 3 a where aone-dimensional view of a device 40 allowing to carry outangle-dispersive X-ray reflectometry measurements is presented.

This device 40 comprises:

-   -   Means 41 for generating and conditioning X-rays. These means        comprise an X-ray source and an optical assembly for        conditioning the beam of X-rays issued from said source, the        optical assembly allowing conditioning in a desired manner a        beam of X-rays which is to be directed on a sample 42,    -   and an X-ray detector 43.

The conditioning carried out by the optical assembly of the means 41corresponds to a controlled dispersion of the incidences of the X-raybeam directed onto the sample.

It is thus sought that the X-rays arrive onto the sample with an angulardispersion of a few degrees.

According to a preferred application of the invention, it is sought toobtain an angular dispersion of the order of 2° or more.

The reflected beam at the sample is then collected through the detector43.

It is specified that the optical conditioning carried out by the opticalassembly of the means 41 can correspond to a one-dimensional effect(e.g. focalization along only one dimension), or a two-dimensionaloptical effect.

Generally the detector 43 is of the PSD type (“Position SensitiveDetector”) and comprises a sensor 430 of the CCD or photodiode type witha large number of pixels.

In the case of the invention and generally of two-dimensional optics,detector 43 can be a two-dimensional detector.

A two-dimensional detector allows to identify and to regroup pixelscorresponding to identical values of incidence angles.

And this type of detector is of particular interest as pixels located atdifferent horizontal positions (the horizontal direction being hereindefined as the direction orthogonal to the plane of FIG. 3 a) cancorrespond to identical angles of incidence.

Indeed a certain divergence along this second dimension (orthogonal tothe plane of the figure) generates a weak variation of the angle ofincidence of X-rays on the sample.

Divergences of the order of 1° can thus be tolerated along this seconddimension in the case of the applications concerned by the invention.

This type of two-dimensional detector thus allows to advantageously usetwo-dimensional optics and particularly optics which allow an importantflux to be collected along the two dimensions which is the case of theoptical assembly of the invention.

In order to perform a two-dimensional conditioning of the beam forangle-dispersive X-ray reflectometry measurements, it is known accordingto a first variant to exploit the diffraction of the beam issued from anX-ray source from an optical assembly whose surface is a crystal curvedalong two dimensions.

Such crystals allow conditioning an initial beam through an X-raydiffraction phenomenon which takes place according to Bragg's law.

It is reminded that the Bragg condition for a crystal is of the formnλ=2d sin θ_(B), where n is the order of reflection, λ the wavelength ofthe incident radiation for which the diffraction occurs, d the spacingperiod between the atomic planes of the crystal which are implied intothe diffraction and θ_(B) the angle of incidence on these same atomicplanes which is necessary for the diffraction to occur.

If one considers an X-ray incident beam, the rays having wavelengths λ,which hit the crystal with an angle of incidence θ_(B) well defined withrespect to a certain family of atomic planes of the crystal shall bediffracted onto these same atomic planes if the Bragg condition hereabove reminded is met.

Thus crystals curved along two dimensions make it possible to produce atwo-dimensional effect on the initial beam, in order to perform theconditioning sought.

This conditioning can thus correspond to a focalization along twodifferent directions.

A particularity of crystals compared to multilayer coating is that it isdifficult to apply on such crystals a gradient in order to increase theeffective area of the crystal.

In this respect, reference can be made to:document “Approaching realX-ray optics”, Hildebrandt et al., Rigaku Journal, Vol. 17 No. 1/2000,(pages 18 to 20 in particular).

This results in that a crystal is limited concerning the flux ofdiffracted X-rays, along the direction of the optical assembly formed bythe crystal for which the angles of incidence of the incident rays onthe crystal vary greatly (the collection surface is limited due to theabsence of gradients).

This direction corresponds to the meridional direction of the opticalassembly formed by the crystal.

According to a second variant of the method of angle-dispersive X-rayreflectometry consisting in performing a conditioning of a beam by anoptical assembly producing a two-dimensional effect, it is also known touse the reflection of an initial beam coming from an X-ray source on anoptical assembly of the “side-by-side” Kirkpatrick-Baez device: type,such as disclosed in U.S. Pat. No. 6,041,099.

Each one of the two mirrors of the KB device contains more preferably alaterally graded multilayer coating, allowing the initial beam X1 to bereflected according to Bragg's law.

We shall return to the definition of laterally graded multilayer.

These optical assemblies of the side-by-side KB type thus make itpossible to condition an initial beam.

But as it will be shown in more detail further on in this text, suchoptical assemblies can be associated to relatively important dimensions.

This naturally constitutes a limitation in these known devices.

It thus appears that the known solutions in order to perform R(θ)measurements for applications for angle-dispersive X-ray reflectometryall contain limitations.

This is among others the case when the dispersion sought in angles ofincidence on the sample is greater than 2° for focalization distancesgreater than 150 mm, and that the collected flux must be important(angular dispersion of about 1° along the direction that is transversalto the general direction of propagation of the rays).

Another goal of the invention is to make it possible to overcome theselimitations.

So as to reach the above-mentioned goals, the invention proposesaccording to a first aspect an optical assembly with a laterally gradedreflective multilayer whose reflecting surface is for reflectingincident X-rays under low incidence angles while producing atwo-dimensional optical effect, characterized by the fact that saidreflecting surface is comprised of a single surface, said reflectingsurface being shaped according to two curvatures corresponding to twodifferent directions.

Preferred aspects, but which are non limitative of such an opticalassembly are the following:

-   -   the lateral gradient extends along the meridional direction of        the incident X-rays,    -   the reflecting surface is smooth,    -   the two-dimensional optical effect is obtained by a single        reflection of incident rays on the optical assembly,    -   said different directions correspond respectively to the sagital        direction and to the meridional direction of the incident        X-rays,    -   the multilayer is a depth-graded multilayer,    -   said reflecting surface is adapted to reflect rays of Cu—Kα        peaks,    -   a first of said two curvatures defines a circle,    -   a first of said two curvatures defines a curve different from a        circle,    -   a first of said two curvatures defines an ellipse or a parabola,    -   a first of said two curvatures defines an open or closed curve        different from a circle, an ellipse or a parabola,    -   the second of said two curvatures defines a circle,    -   the second of said two curvatures defines a curve different from        a circle,    -   the second of said two curvatures defines an ellipse or a        parabola,    -   the reflecting surface has a geometry of substantially toroidal        shape,    -   the reflecting surface has a geometry of substantially        paraboloidal shape,    -   the reflecting surface has a geometry of substantially        ellipsoidal shape,    -   the reflecting surface has a geometry substantially circular in        shape along a first direction, and elliptic or parabolic along a        second direction,    -   the reflecting surface has a sagital curvature radius of less        than 20 mm,    -   a window that is opaque to X-rays and containing an aperture is        associated at the input and/or output of the optical assembly,        in order to control the input and/or output flux of the optical        assembly,    -   the windows are removable,    -   the assembly comprises an aperture located at the input        cross-section and the size and the shape of said aperture        located at the input cross-section can be adjusted in order to        control the incident flux,    -   the assembly comprises an aperture located at the output        cross-section and the size and the shape of said aperture        located at the output cross-section can be adjusted in order to        control the reflected flux,    -   the apertures of the windows are dimensioned in order to realize        a flux/divergence compromise of the radiation.

According to a second aspect, the invention also proposes amanufacturing method of an optical assembly according to one of theabove aspects, characterized in that the method includes the coating ofa substrate already having a curvature, and the curvature of thissubstrate along a second different direction.

Preferred aspects, but which are non limitative of this manufacturingmethod are the following:

-   -   the direction along which the substrate already has a curvature        corresponds to the sagital direction of the optical assembly,    -   said curvature of the substrate which corresponds to the sagital        direction of the optical assembly defines a radius of curvature        which is less than 20 mm,    -   the direction along which the substrate is curved corresponds to        the meridional direction of the optical assembly,    -   said substrate has a roughness lower than 10 rms,    -   the substrate itself is constituted, starting from an element in        the form of a tube, cone, or pseudo-cone already having a        curvature along a direction orthogonal to the axis of the tube,        of the cone or of the pseudo-cone,    -   the element is a glass tube with a circular transversal        cross-section,    -   the glass is of the Duran type (registered trademark),    -   the constitution of the substrate includes the cutting of the        tube along the longitudinal direction of the tube, in such a way        as to obtain a substrate in the shape of an open cylinder,    -   the cutting along the longitudinal direction of the tube is        followed by cutting in order to dimension the optical assembly        in length,    -   the coating is performed in order to constitute a multilayer        before curving the substrate,    -   the substrate is curved in order to conform it to the geometry        sought before coating it in order to constitute a multilayer,    -   the optical assembly is coupled to a filter, in order to provide        attenuation of the undesired spectral bands while guaranteeing        sufficient transmission of a predetermined wavelength band for        which reflecting the incident X-rays is sought,    -   the filter is a 10-μm Nickel filter,    -   the filter is realized by one of the following techniques:        -   realization of two filters whose combined thickness            corresponds to the filter thickness sought, positioned            respectively on the input and output windows of the            radiation of a protective housing containing the optical            assembly,        -   deposit of a layer of filtering material on the multilayer            coating, with a coating thickness that is approximately            given by the following relationship: d=(e sin Θ)/2 (where e            is the required filter “optical” thickness and θ the angle            of incidence on the optic).

And according to a third aspect, the invention proposes a device forgenerating and conditioning X-rays for applications for angle-dispersiveX-ray reflectometry including an optical assembly according to one ofthe above aspects connected to a source of X-rays in such a way that theX-rays emitted by the source are conditioned along two dimensions so asto adapt the beam emitted by the source in destination of a sample, theX-rays having different angles of incidences on the sample which isconsidered.

Preferred aspects, but which are non limitative of such a device are thefollowing:

-   -   the dispersion of incidence angles on the sample corresponds        substantially to the angular dispersion along the sagital        dimension of the beam reflected by the optical assembly,    -   the optics are directed with respect to the sample so that the        normal at the center of the optical assembly is approximately        parallel to the surface of the sample,    -   the capture angle at the level of the sample is greater than 2°        along a first dimension corresponding to the sagital dimension        of the optical assembly and about 1° along a second dimension        corresponding to the meridional dimension of the optical        assembly, the optical assembly being positioned so that the        dispersion of incidence angles of the X-rays on the sample is        greater than 2°, the sample being placed at a distance greater        than 15 cm in relation to the optical assembly.

Other aspects, goals and advantages of the invention shall appearclearer after the following disclosure of preferred embodiments of theinvention, reference being made to the annexed drawings in which, inaddition to FIGS. 1 a, 2 a and 3 a which have already been coveredabove:

FIG. 1 is a block representation of a first embodiment of an opticalassembly according to the invention, allowing to perform atwo-dimensional focalization of an incident beam of X-rays,

FIG. 2 is an analogous view showing a second embodiment of an opticalassembly according to the invention, allowing to perform a collimationof an incident beam of X-rays,

FIG. 3 is an analogous view showing a third embodiment of an opticalassembly according to the invention, wherein a low divergence of thereflected flux is sought,

FIG. 4 is a block representation of an angle-dispersive X-rayreflectometry device according to the invention (with the X-ray detectornot shown in this figure for reasons of clarity),

FIGS. 5 a and 6 a highlight in a schematic manner the elongationconstraints, associated with KB optical assemblies of a known type,which are required to increase the angular dispersion of the reflectedbeam along the directions that are transversal to the direction ofpropagation of the beam.

It is specified in the preamble of this disclosure that the figures aremeant to show the principle of the invention, and do not necessarilyshow the dimensions and scales in a realistic manner.

This is true in particular for the angles of incidence (or even anglesof reflection) of X-rays.

These X-rays arrive actually on the reflecting surfaces according to theinvention with an incidence of less than 10°.

The meridional and sagital directions are also defined with respect tothe general direction of propagation of the beam of X-rays:

-   -   The meridional direction corresponds to the average direction of        propagation of this beam (and more precisely to the average        direction between the average directions of propagation of the        beam before and after its reflection on the optical assemblies        to be described),    -   The sagital direction corresponds to an horizontal direction        that is transversal to this meridional direction (with the        vertical here being defined as the average normal to the portion        of the reflecting surface of the optical assemblies which shall        be described and which is effectively used to reflect the        incident beam of X-rays).

DISCLOSURE OF THE OPTICAL ASSEMBLY BEING CONSIDERED IN THE INVENTION

Now referring to FIG. 1, an optical assembly 10 designed to reflectincident X-rays coming from a source S of X-rays is shown.

Source S can be in particular of the X-ray tube, rotating anode, ormicro-focus X-ray source type.

Optical assembly 10 includes a multilayer structure formed on asubstrate (e.g. glass), that defines a reflecting surface for theincident X-rays.

The reflecting surface of this optical assembly has a particulargeometry.

More precisely, this reflecting surface is shaped according to twocurvatures corresponding to two different directions.

And this reflecting surface thus shows important differences withrespect to reflecting surfaces of the type of those used in opticalassemblies such as those covered in U.S. Pat. No. 6,041,099:

-   -   The reflecting surface is a single reflecting surface, in        opposition to what is the case for optical assemblies in which        two different elementary mirrors were put together,    -   This reflecting surface is smooth (this term meaning in the        present text that the reflecting surface does not present any        second degree discontinuity (crests or angular points—salient or        hollow—etc.),    -   Moreover, a difference that is also important is that in the        case of the invention, the incident rays only undergo a single        reflection in order to produce the desired two-dimensional        optical effect, while two reflections are required in the case        of the optical assembly disclosed in U.S. Pat. No. 6,041,099.

Even more precisely, the reflecting surface of the optical assemblyaccording to the invention has a curvature Rx in the meridionaldirection X, and a curvature Ry in the sagital direction Y.

FIG. 1 shows these curvature radiuses, two curves Cx and Cy being shownin order to show the appearance of the curves defined by the respectivecurvature radiuses Rx and Ry.

Each one of the two curvature radiuses can be constant, or vary alongits associated curve.

Each one of curves Cx, Cy can thus be a circle, but also an ellipse, aparabola, or another curve (open or closed).

In any case, the reflecting surface of the optical assembly 10 does nothave a simple spherical shape (i.e. the radiuses Rx and Ry are notsimultaneously equal and constant).

Each one of curves Cx, Cy is thus associated to a different direction inspace (two orthogonal directions on the example covered here).

And each one of these curves produces on the X-rays that are reflectedon the reflecting surface a one-dimensional optical effect:

-   -   Curve Cx produces a one-dimensional optical effect along        direction X,    -   Curve Cy produces a one-dimensional optical effect along        direction Y.

And each one of these dimensional effects depends on the curvatureassociated with the curve, and on the evolution of said curvature alongthis curve.

Curves Cx and Cy can thus have their parameters set in order toselectively obtain associated one-dimensional effects such asone-dimensional collimation or focalization.

FIG. 1 shows the case in which each curve Cx, Cy produces aone-dimensional focalization.

To this end, Rx and Ry are different, but each one is constant (curvesCx and Cy are circles).

In this preferred embodiment, the reflecting surface of the opticalassembly thus has a toroidal geometry.

This produces a two-dimensional focalization, which concentrates thedivergent rays coming from source S to a single image point I.

And curvature radius Ry (sagital curvature radius) can have (in thisembodiment as in the others) a value of less than 20 mm required forfocalizations over short distances, less than 90 cm (source-focalizationpoint distance) according to a preferred application of the invention.We shall return to this aspect.

One will note that the optical assembly according to the invention makesit possible to overcome the drawbacks mentioned in the introduction ofthis text concerning mirrors of the multilayer coating <<Montel>> type.

In particular, as has already been mentioned above, this opticalassembly is in a single piece (it does not require delicate assembly).

And the incident X-rays only undergo a single reflection on itsreflecting surface.

Furthermore, the reflecting surface is single and smooth.

It has been stated that the reflecting surface of optical assembly 10was defined by a multilayer.

This multilayer (as with all the multilayers covered in this text)includes a minimum of one <<lateral gradient>>.

This characteristic makes it possible to efficiently reflect X-rays thatpresent different local incidences with respect to the reflectingsurface.

It shall be understood indeed that the different areas of the reflectingsurface do not receive the incident X-rays with the same local incidence(due to the divergence of the incident beam, and to the geometry of thisreflecting surface).

By laterally graded multilayer is meant here a multilayer whose layerstructure is adapted so that the Bragg condition is respected at allpoints of the effective area of the mirror.

It is reminded that the Bragg condition is of the form nλ=2d*sin θ,with:

-   n: order of reflection,-   λ: wavelength of the incident radiation,-   d: period of the multilayer,-   θ: angle of incidence on the surface of the multilayer.

Thus, for radiation of incident X-rays along a narrow wavelength bandcontaining copper Kα lines for example (Cu—Kα peaks of wavelengths ofapproximately 0.154 nm), the laterally graded multilayer mirror allowsthe Bragg conditions to be maintained across the entire effective areaof the mirror.

This causes the predetermined wavelength band to be reflected (in theabove example containing the Copper Ka peaks), by different regions ofthe mirror on which the incident rays have variable local angles ofincidence.

Thus the surface of the mirror that is effectively used can beincreased.

The gradient is obtained by varying the period of the multilayerlocally, in an adapted manner.

This type of laterally graded multilayer structure thus allows the solidcollection angle of the optical assembly to be increased, leading to areflected flux that is higher with respect to monolayer mirrorsoperating by total reflection, for an identical optical geometry.

The presence of a lateral gradient also makes it possible to overcomethe limits of certain known configurations, such as configurations usingthe Rowland circle for which the distance between the source and theoptics and the distance between the optics and the sample are identical,and the variations in angle of incidence on the optics can be slight foroptics of small size.

Indeed, configurations using the Rowland circle allow gradientlessoptics to be used but have the limitation of not being able to performan enlargement or reduction of the image area in relation to the source(a reduction in the image area compared to the source can be consideredthrough the usage of slits, but this mean is not very accurate andlimits the collected flux).

An illustration of this type of known configuration can be found indocument <<A point-focusing small-angle x-ray scattering camera using adoubly curved monochromator of a W/Si multilayer>> by Sasanuma and al.(Review of Scientific Instruments, American Institute of Physics, NewYork vol. 67 No. 3, Mar. 1, 1996 (pages 688–692)).

It is specified that the multilayer of the different embodiments of theinvention can also present a depth gradient (gradient in the thicknessof the multilayer).

Such a depth gradient allows the Bragg conditions to be fulfilled forfixed angles of incidence and variable wavelengths, or vice-versa.

It is thus possible for example to increase the width of the pass bandin wavelength of the multilayer of the optical assembly, and to focus orcollimate X-rays of different wavelengths, at the level of a same givenimage plane (case of a fixed geometry—i.e. a configuration wherein therelative positions of the source of the incident rays, of the opticalassembly and of the image plane are fixed).

Sources of X-rays with different wavelengths can in this way be used toreflect the X-rays coming from the different sources with the sameoptical assembly, without requiring the source and/or the imagesplane(s) to be positioned again in relation to the optical assembly.

In this case the wavelength tolerance of the optical assembly is used(tolerance in Δλ).

In the same way, it is also possible to translate this tolerance in Δλinto a tolerance in Δθ.

A tolerance on the wavelength corresponding indeed—within the frameworkof the Bragg condition—to a tolerance on the angle of incidence, it ispossible at a constant wavelength for the incident beam to collect andto reflect an incident luminous flux whose rays of the same wavelengthhave different local incidences.

In particular, X-ray sources of a larger dimension can in this way beused (increase in the angular acceptance).

Referring now to FIG. 2, another preferred embodiment of the inventionis shown, illustrated by optical assembly 20.

The reflecting surface of the multilayer of this optical assembly isshaped in the respective directions X and Y along two curves Cx and Cyrespectively parabolic and circular, each one of these curves producinga collimation along its associated X or Y direction.

A parallel collimation along all the directions in space is thusgenerated from the divergent incident beam.

And it is thus possible to realize according to the invention opticalassemblies comprised of a multilayer mirror (laterally graded, andpossibly furthermore with a depth gradient), whose reflecting surfacecan have one from among any different aspheric complex shapes.

It is thus possible in particular to give this reflecting surface one ofthe following geometries:

-   -   geometry of substantially toroidal shape,    -   geometry of substantially paraboloidal shape,    -   geometry of substantially ellipsoidal shape,    -   geometry of substantially circular shape along a first direction        (the sagital direction in particular), and elliptic or parabolic        along a second direction (the meridional direction in        particular).

The lateral gradient can in particular extend along the meridionaldirection of the incident X-rays.

And the period of the multilayer can be adapted in order to reflect inparticular the rays of the Cu—Kα peaks.

Referring now to FIG. 3, an optical assembly 30 according to theinvention is shown, equipped with two end walls 31 and 32, positionedrespectively at the input cross-section and at the output cross-sectionof the radiation that must be reflected by this optical assembly.

Each wall 31, 32 has an aperture (respectively 310, 320) allowing theX-ray radiation to pass, with the walls themselves being opaque toX-rays.

The walls can be made of lead, for example.

And it is possible to adjust the shape and the size of each aperture(independently of the other aperture), in order to control the incidentflux (via the aperture located on the input cross-section), and thereflected radiation (via the aperture located on the outputcross-section).

The apertures can thus be dimensioned, in order to find a compromisebetween the intensity of the flux (as input or output), and itsdivergence.

It is specified that walls 31 and 32 can be designed to be removable,for example being screwed onto the horizontal transversal edges of theoptical assembly, as shown in FIG. 3.

In this way, optical assemblies can be adapted in a flexible manner inorder to find if needed a desired flux/divergence compromise.

It is also possible to consider solely an input wall, or an output wall.

And each wall associated to its aperture thus constitutes a <<window>>allowing the X-rays to pass.

DISCLOSURE OF A PREFERRED MANUFACTURING METHOD

A preferred method shall now be described, allowing an optical assemblyof the type disclosed above to be obtained, while obtaining thefollowing advantages:

-   -   guarantee a very good surface condition of the substrate used to        perform the multilayer coating (the surface roughness        specifications for substrates of X-ray multilayer mirrors        correspond normally to degrees of roughness that must not exceed        a maximum value of about 10 angstroms rms (root mean square)),        and    -   while also allowing surfaces to be constituted along an        extremely reduced sagital curvature radius Ry, of a value for        example that is less than 20 mm (making focalization possible        for example along a source—focalization point distance of less        than 90 cm).

Indeed, it would be difficult to obtain a substrate surface for thecoating of the multilayer having such radius curvature values andsurface condition:

-   -   by implementing the polishing of a substrate that already has        such a low sagital curvature: in this case the polishing of the        pre-shaped substrate is delicate,    -   or by curving along the sagital curvature radius Ry a flat        substrate that is already polished—in this case it would be        difficult to obtain the low curvature radiuses sought (while        such curvature radiuses allow the optical effects sought to be        produced over short distances, and to thus reduce the space        occupied by the optical assembly).

In the case of the manufacturing method according to the invention, thesurface condition sought is obtained without any special treatment, byusing in order to form the optical assembly a substrate that already hasa curvature along a direction of curvature.

And the direction along which the substrate already has a curvaturecorresponds more preferably to the sagital direction of the opticalassembly, once the latter has been manufactured and positioned withrespect to the X-ray source (this direction being as it has already beenmentioned defined with respect to the incident radiation, but being alsopossibly defined in relation to the optical assembly itself to theextent where the optical assembly is to be directed in a specific waywith respect to the incident radiation).

Such a substrate has a face corresponding to the face of the opticalassembly that shall carry the reflecting surface. This face of thesubstrate shall be called <<optical face>>.

Thus, generally according to the invention a substrate is used thatalready has a curvature (along a direction that shall be made tocorrespond more preferably to the sagital direction of the opticalassembly), and this substrate is curved along a second direction whichis different (corresponding more preferably to the meridional directionof the optical assembly).

A coating of the optical face of the substrate with a multilayer is alsoperformed. This coating can be performed before the curving of thesubstrate, or after.

In any case, an optical assembly is obtained in this way.

By selecting a substrate having the desired curvature (in form and invalues(s) of curvature radius (es)), and by curving it as desired, anoptical assembly having the geometry sought can be obtained.

It is also possible to constitute the substrate itself, in particularusing an element (in particular made of glass) such as a tube, cone, oreven a pseudo-cone (which is defined here as a surface of revolutiongenerated by the revolution along a curve such as an ellipse of agenerating straight line that is oblique with respect to its axis ofrevolution and cuts the latter in space).

In the case of an element in the form of a tube, the tube can have atransversal cross-section which is circular, but also elliptic, orcorrespond to any closed curve.

And such an element can also be an open cylinder whose directrix is anopen curve such as a parabola segment.

In any case, the starting element has a curvature along a directioncorresponding more preferably to the sagital direction of the opticalassembly for which manufacture is sought.

And this direction is orthogonal to the axis of the tube, of the cone orof the pseudo-cone.

In a preferred embodiment, such a substrate can in particular beobtained using a glass tube whose transversal cross-section is circular.

In this preferred embodiment, the substrate from which the opticalassembly is to be manufactured and which has a curvature along adirection can be in particular obtained by:

-   -   cutting a glass tube having the sagital curvature radius sought,        such as a glass tube of the Duran type (registered trademark)        manufactured by the SCHOTT company, then    -   coating the tube thus cut with successive coatings of material        in order to constitute the multilayer on top.

Such a substrate shall then be curved along a direction (preferablymeridionally), with the curvature sought, in order to obtain the opticalassembly.

And it is specified that it is possible—in this embodiment as in theothers—to first proceed with curving the element (here the cut tube),and with the coating afterwards.

It is specified that in all of the embodiments for implementing themethod according to the invention, the multilayer thus made is alaterally graded multilayer (and possibly also with a verticalgradient).

Cutting the glass tube is performed along the longitudinal direction ofthe tube by performing a cross-section along a direction that isparallel to the axis of symmetry of the tube (and which may even includethis axis in order to constitute a half-tube), so as to obtain asubstrate in the form of an open cylinder.

The directrix of this open cylinder thus has in this preferredembodiment the shape of a portion of a circle—e.g. a half-circle.

This longitudinal cutting is followed by another cutting in order todimension the optics in length.

After these cutting operations, a substrate has thus been constitutedfor the manufacture of an optical assembly according to the invention.

After having coated the substrate with the multilayer, the coatedsubstrate is curved along the desired second direction, whichcorresponds to the meridional direction, in order to conform the surfaceof the multilayer according to the desired geometry.

Thus, in this preferred embodiment of the manufacturing method of theoptical assembly according to the invention, a cylindrical substrate canbe constituted with a directrix having approximately the shape of aportion of a circle, then the coating of such a substrate is performed,and the curving of this substrate along a direction that is not includedin the plane of the directrix of the cylinder of said substrate (inparticular along the direction of the generatrix of the cylinder).

The Applicant has observed that it was thus much easier to manufactureoptical assemblies according to the invention, than by one of theabove-mentioned techniques (coating of a substrate that is alreadyentirely conformed to the geometry sought, or curvature along twodirections of a planar multilayer).

And it is thus possible to obtain substrates, used afterwards formultilayer coating, with a very good surface condition (roughness notexceeding 10 angstroms rms), and low sagital curvature radiuses (lessthan 20 mm).

The optical effects can thus be obtained over short distances.

In the case of two one-dimensional focalizations, it is thus remindedthat the characteristics of tangential (meridional) and sagitalfocalization are given by the following formulas, for a toroidal mirror:

-   -   tangential focalization: 1/p+1/q=2/(Rx sin θ) with p:        source-mirror distance, q: mirror-focalization plane distance,        θ: Angle of incidence,    -   sagital focalization: 1/p+1/q=2 sin θ/Ry (for focalization along        two dimensions, the p and q distances are identical for the two        formulas).

It is specified that it is also possible as a variant to start with thesame part of a cut tube in order to form a cylinder whose directrix isopen, and to inverse the order of coating and of curving in the seconddirection in relation to what was disclosed above.

In this case, first the cylindrical substrate is curved, then coating isperformed in order to constitute the multilayer on the thus conformedsurface.

In any case, coating can be carried out with any type of material thatallows reflective multilayers for X-rays to be performed.

And this coating can use any type of known method for this purpose, e.g.sputtering (possibly assisted by plasma) or another type of coating in avacuum.

It is also specified that for applications that require a high degree ofspectral purity, the optical assembly for reflecting X-rays can beconnected to a filter manufactured using appropriate material andthickness, in order to provide attenuation of the undesired spectralbands while guaranteeing sufficient transmission of a predeterminedwavelength band for which reflecting the incident X-rays is sought.

Thus for optics made with W/Si multilayer coatings to reflect the CopperKa peaks, a 10-μm Nickel filter can be used to attenuate the copper Kβpeak (0.139 nm) by a factor of 8 while maintaining sufficienttransmission for the Kα peaks (greater than 60%).

This filtering function comes in addition to the “naturalmonochromatization” obtained using the multilayer and can thus make itpossible for applications where spectral purity is a priority toincrease the performance of the multilayer optics disclosed in theinvention.

Concerning this aspect, two alternative embodiments of the filter are tobe considered:

-   -   realization of two filters whose combined thickness corresponds        to the filter thickness sought (e.g. two filters having the same        thickness that is equal to half of the total thickness sought),        positioned respectively on the input and output windows of the        radiation of a protective housing containing the optical        assembly,    -   coating with a layer of material (used for filtering) on the        multilayer coating. The optics surface is then comprised of a        reflecting multilayer coating (laterally graded) and of a        surface layer providing the function of a filter in order to        increase the spectral purity of the reflected radiation. The        thickness coated is thus given approximately by the following        relationship: d=(e sin θ)/2 (where e is the required filter        “optical” thickness and θ the angle of incidence on the optics).

DISCLOSURE OF A PARTICULAR APPLICATION MODE

An aspect of the invention is now going to be disclosed that concernsmore particularly angle-dispersive X-ray reflectometry.

FIG. 4 shows a device 60 which allows measurements of the R(θ) type tobe performed for this type of application.

More precisely, in this figure are shown:

-   -   A source S of X-rays,    -   an optical assembly 61 for conditioning initial beam X1 issued        from source S,    -   and a sample 62.

The X-ray detector normally designed for detecting the rays coming fromthe reflection on the sample is not shown in this figure, for reasons ofclarity.

In reference to FIG. 4, it is specified that the angular dispersions ofreflected beam X2 on optical assembly 61 are not representative.

Indeed in FIG. 4, the angular dispersion (β_(M)) along the meridionaldirection appears greater than the angular dispersion along the sagitaldirection (β_(S)). This is not the case according to an advantageousalternative of the invention.

Device 60 contains a source S of X-rays that emits initial beam X1.

Initial beam X1 issued from the source is directed to optical assembly61, whose reflecting surface is conformed along two curvaturescorresponding to two different directions.

This optical assembly 61 is thus capable of producing on initial beam X1a two-dimensional optical effect, in order to generate beam X2 which hasa controlled angular dispersion.

Beam X2 is then directed to sample 62 for which characterization ofreflectivity is sought, e.g. for applications such as mentioned at thebeginning of this text concerning R(θ) type measurements.

The different elements of device 60 are fixed for all of the R(θ)measurements for a given analysis area on the sample.

And optical assembly 61 allows generation of beam X2 conditioned asdesired according to a two-dimensional effect (which is typically atwo-dimensional focalization).

More precisely, optical assembly 61 conditions beam X2 so as to obtainhigh convergence angle at the level of the sample, and in particularalong a dimension corresponding to the sagital dimension of opticalassembly 61 (i.e. direction Y in FIG. 4).

Even more precisely, according to an advantageous alternative of theinvention:

-   -   the capture angle at the level of the sample (i.e. the        convergence angle of the optic) is:        -   greater than 2° along a dimension (corresponding to the            sagital dimension of the optic),        -   and about 1° along another dimension (corresponding to the            meridional dimension of the optic),    -   and the dispersion of the angles of incidence of the rays of        beam X2 on the sample is greater than 2°, the sample being        placed at distances greater than 15 cm in relation to the        optical assembly.

This is obtained in particular:

-   -   by the geometry of the surface of optical assembly 61,    -   by the positioning of this optical assembly in relation to        sample 62: this positioning is defined so that the dispersion of        the angles of incidence of the X-rays arriving on the sample is        greater than 2°.

This device 60 allows quick measurements of angle-dispersive X-rayreflectometry to be taken, as it does not imply the displacement of anymechanical element.

It is indeed the optical assembly 61 that provides the angulardispersion of beam X2 by conditioning this beam so as to adapt it to thelevel of the sample so that the X-rays arriving on this sample havedifferent angles of incidence at the level of the image area which isconsidered (focal point of optical assembly on the sample).

Optical assembly 61 thus has a single reflecting surface, this surfacebeing curved along two dimensions with a first curvature along thesagital direction and a second curvature along the meridional direction.

FIG. 4 shows a more detailed illustration of this optical assembly.

It is in this case an optical assembly allowing focalization in twodimensions to be performed with a first curvature along direction Y(circular curvature CY) and a second curvature along direction X(circular curvature CX).

In this precise case, the optics thus have a toroidal shape.

Generally, optical assembly 61 can have a toroidal shape or anellipsoidal shape in the case of a two-dimensional focalization.

Optical assembly 61 can also have a paraboloidal shape in the case of atwo-dimensional collimation.

According to another variant, optical assembly 61 can also have acircular curvature along one dimension, by way of example along, thesagital direction, and a parabolic curvature along another dimension, byway of example along the meridional direction.

Optical assembly 61 has a laterally graded multilayer coating (i.e.along the meridional direction corresponding to direction X in FIG. 4).

Note that additional elements can be positioned upstream of opticalassembly 61 (between source S and this optical assembly) such as forexample slits in order to adjust the beam.

Optical assembly 61 has a large effective collection area, allowing ahigh convergence angle to be obtained at the level of the sample inparticular along the sagital dimension of the optics.

By way of example, the effective collection area of optical assembly 61can thus also have along the sagital direction a dimension of about 1 cmfor focalization distances of about 200 mm. The dimension referred toabove corresponds to the length of the straight line obtained byconnecting the two end points of the effective collection area along thesagital direction.

Thus for an optical assembly having a curvature radius of about 7 mm(and focalizing the beam using a source placed at 40 cm) the effectivecollection area can correspond to a portion along the sagital dimensionof approximately one quarter of a circle, i.e. about 1 cm, whichcorresponds to a capture angle at the level of the sample of about 3°.

According to an advantageous alternative of the invention, opticalassembly 61 thus allows a capture angle to be obtained at the level ofthe sample which is:

-   -   greater than 2° along a first dimension of optical assembly 41        (corresponding to its sagital direction, i.e. direction Y in        FIG. 4),    -   about 1° along a second dimension of optical assembly 61        (corresponding to its meridional direction, i.e. direction X in        FIG. 4).

In such a configuration, the sample is positioned at focalizationdistances (distance between optical assembly 61 and the sample) that aregreater than 150 mm.

By way of example, the focalization distances can be about 300 mm to 200mm.

The orientation of optical assembly 61 shall thus be adapted in relationto the sample in such a way that the dispersion of angles of incidenceof the X-rays on the sample is greater than 2°.

The orientation of optical assembly 61 is defined as the angularposition of this optical assembly for a given rotation around itsoptical axis (axis parallel to the meridional direction).

A preferred positioning of the elements of the device consists indirecting the optical assembly in such a way that the dispersion ofangles of incidence on the sample corresponds substantially to theangular dispersion along the sagital dimension (direction Y in FIG. 4)of reflected beam X2 at the level of the optical assembly.

A preferred positioning thus consists in directing the optical assemblyin such a way that the average normal to the effective area of theoptical assembly (or the normal at the center of the optic) isapproximately parallel to the surface of the sample.

For the field of application of the invention, the average incidences onthe sample are low and in the case of very low incidences (averageincidence of about 1°) the orientation of optical assembly 61 can bedescribed as being such that:

-   -   the average normal of the optical assembly is approximately        parallel to the surface of sample 62,    -   the sagital direction of optical assembly 61 is approximately        orthogonal to the surface of sample 62,    -   the meridional direction of optical assembly 61 is approximately        parallel to the surface of sample 62.        An illustration of this type of setup is shown in FIG. 6.

In any case according to a preferred application of the invention,optical assembly 61 shall not be directed in such a way that the averagenormal to the effective area of optical assembly 61 is approximatelyorthogonal to the surface of sample 62 if one considers a low incidence(the dispersion of angles of incidence on sample 62 would thencorrespond substantially to the angular dispersion of beam X2 along themeridional direction).

Along the second dimension of the optical assembly, i.e. the meridionaldirection, the optical assembly allows an important flux to be collectedand according to a preferred application, the angular dispersion ofreflected beam X2 is about 1° along this meridional direction (directionX in FIG. 4).

Optical assembly 61 thus allows a high dispersion of angle incidences tobe obtained at the level of the sample while conditioning a maximum offlux at the level of the sample.

Note that compared to a configuration in which one would use as opticalassembly producing a two-dimensional effect an optical assembly of theside-by-side KB type, the invention allows more compact devices to berealized.

Optical assembly 61 indeed allows for a given length (along themeridional direction) to obtain a bigger collection surface along thesagital direction than what would be obtained with a configuration usinga conditioning via an optic of the KB type.

Thus, in the case of the invention the angular dispersion of the beamtreated by the effective area of the optical assembly is greater alongthe sagital direction, and high angular dispersion on the sample isobtained.

As an example and in reference to FIGS. 5 a and 6 a, obtaining anequivalent angular dispersion with optical assemblies of the KB typewould require the elongation of the optical assembly along direction Y.

In the case of optical elements of the KB type, any incident ray mustindeed hit the optical assembly in a particular zone (corresponding tothe hatched zones of the mirrors in FIGS. 5 a and 6 a) to undergo doublereflection.

This therefore results in that for such a known type of optical element,the solid angle that can be collected is limited by the length of theoptical assembly.

And this is true for horizontal transversal directions as well as forvertical transversal directions (respectively direction Z or direction Xin FIGS. 5 a and 6 a).

In the case of the invention, it is therefore possible to increase theeffective collection area along the sagital direction, withoutincreasing the length of the device.

This is important among others in the case where it is desired to limitthe amount of space occupied and therefore the size of the optics, as itis the case in the field of application of the invention.

By way of example, in the case where optical assembly 61 has a toroidalsurface geometry, the effective collection area of the mirror along thesagital dimension can describe a portion such as a quarter or even ahalf-circle, corresponding to a capture angle at the level of the samplealong the sagital dimension that is important.

The possibility for optical assembly 61 to increase the effectivecollection area along the sagital direction is due to the fact that theangle of incidence on the optics of the X-rays coming from a same pointsource varies very little along this direction (direction Y in FIG. 4).

1. An optical assembly comprising a laterally graded reflectivemultilayer having a reflecting surface to reflect incident X-rays underlow incidence angles while producing a two-dimensional optical effect,said reflecting surface comprising a single surface conformed along twocurvatures corresponding to two different directions; wherein said twodifferent directions correspond respectively to sagital and meridionaldirections of the incident X-rays, and said reflecting surface has asagital curvature radius of less than 20 mm.
 2. The optical assembly ofclaim 1, wherein the laterally graded reflective multilayer extendsalong the meridional direction of the incident X-rays.
 3. The opticalassembly as claimed in claim 1 or 2, wherein the reflecting surface issmooth.
 4. The optical assembly of claim 1, wherein the two-dimensionaloptical effect is obtained by a single reflection of incident rays onthe optical assembly.
 5. The optical assembly of claim 1, furthercomprising a substrate coated with said laterally graded reflectivemultilayer, said substrate having a roughness less than 10 angstromsrms.
 6. The optical assembly of claim 1, wherein the multilayer is adepth graded multilayer.
 7. The optical assembly of claim 1, wherein thereflecting surface is adapted to reflect rays of Cu-Kα peaks.
 8. Theoptical assembly of claim 1, wherein a first one of said two curvaturesdefines a circle.
 9. The optical assembly of claim 1, wherein a firstone of said two curvatures defines a curve different from a circle. 10.The optical assembly of claim 9, wherein the first curvature defines anellipse or a parabola.
 11. The optical assembly of claim 1, wherein afirst one of said two curvatures defines an open or a closed curvedifferent from a circle, an ellipse or a parabola.
 12. The opticalassembly as in any one of claims 8, 9, 10 or 11, wherein a second one ofsaid two curvatures defines a circle.
 13. The optical assembly as in anyone of claims 8, 9, 10 or 11, wherein a second one of said twocurvatures defines a curve different from a circle.
 14. The opticalassembly of claim 13, wherein the second curvature defines an ellipse ora parabola.
 15. The optical assembly as in any one of claims 8, 9, 10 or11, wherein a second one of said two curvatures defines an open or aclosed curve different from a circle, an ellipse or a parabola.
 16. Theoptical assembly of claim 1, wherein the reflecting surface has ageometry of substantially toroidal shape.
 17. The optical assembly ofclaim 1, wherein the reflecting surface has a geometry of substantiallyparaboloidal shape.
 18. The optical assembly of claim 1, wherein thereflecting surface has a geometry of substantially ellipsoidal shape.19. The optical assembly of claim 1, wherein the reflecting surface hasa substantially circular geometry along a first direction and asubstantially elliptic or parabolic geometry along a second direction.20. The optical assembly of claim 1, further comprising at least onewindow that is opaque to X-rays, the at least one window having anaperture therein and being associated with an input or an output of theoptical assembly in order to control a flux of the optical assembly. 21.The optical assembly of claim 20, wherein the at least one window isremovable.
 22. The optical assembly of claim 20, wherein the aperture islocated at an input cross-section, and the size and the shape of saidaperture can be adjusted in order to control an incident flux.
 23. Theoptical assembly of claim 20, wherein the aperture is located at anoutput cross-section, and the size and the shape of said aperture can beadjusted in order to control a reflected flux.
 24. The optical assemblyas claimed in one of claims 20 or 21, wherein the aperture of the atleast one window is dimensioned to realize a flux/divergence compromiseof radiation.
 25. A method of manufacturing an optical assemblycomprising a laterally graded reflective multilayer having a reflectingsurface to reflect incident X-rays under low incidence angles whileproducing a two-dimensional optical effect, said reflecting surfacecomprising a single surface conformed along two curvatures correspondingto two different directions, the method comprising: providing asubstrate having a curvature along a first direction; coating thesubstrate; and curving the substrate along a second direction differentthan the first direction; wherein one of the first or second directionsis a sagital direction of the incident X-rays, and the curvature of thesubstrate corresponding to the sagital direction defines a radius ofcurvature which is less than 20 mm.
 26. The method of claim 25, whereinthe first direction along which the substrate already has a curvaturecorresponds to the sagital direction of the optical assembly.
 27. Themethod as claimed in one of claims 25 or 26, wherein the seconddirection along which the substrate is curved corresponds to ameridional direction of the optical assembly.
 28. The method of claim25, wherein the substrate has a roughness lower than 10 angstroms rms.29. The method of claim 25, wherein providing the substrate comprisesproviding an element in the form of a tube, cone, or pseudo-cone alreadyhaving a curvature along a direction orthogonal to the axis of the tube,of the cone or of the pseudo-cone.
 30. The method of claim 29, whereinthe element comprises a glass tube having a circular transversalcross-section.
 31. The method of claim 30, wherein the glass is of aborosilicate glass 3.3 type.
 32. The method of claim 30, furthercomprising cutting the glass tube along a longitudinal direction so thatthe substrate has a shape of an open cylinder.
 33. The method of claim32, further comprising cutting in order to dimension the opticalassembly in length after cutting the glass tube along the longitudinaldirection.
 34. The method of claim 25, wherein coating the substrate isperformed to achieve a multilayer before curving the substrate.
 35. Themethod of claim 25, wherein the substrate is curved in order to conformit to a predetermined geometry before the coating step.
 36. The methodof claim 25, further comprising coupling the optical assembly to afilter to provide attenuation of undesired spectral bands whileguaranteeing sufficient transmission of a predetermined wavelength band.37. The method of claim 36, wherein the filter comprises a 10-μm nickelfilter.
 38. The method of claim 36, wherein the filter is fabricated byone of: providing a pair of filters to obtain a combined thicknesscorresponding to a predetermined filter thickness, a first one of thepair of filters positioned on an input window and a second one of thepair of filters being positioned on an output window of a protectivehousing containing the optical assembly; or depositing a layer offiltering material on the multilayer, the layer of filtering materialhaving a coating thickness approximately given by the followingrelationship:d=(e sin θ)/2, wherein e is a required filter optical thickness and θ isan angle of incidence.
 39. A device for generating and conditioningX-rays for angle-dispersive X-ray reflectometry, the device comprising:an optical assembly comprising a laterally graded reflective multilayerhaving a reflecting surface to reflect incident X-rays under lowincidence angles while producing a two-dimensional optical effect, saidreflecting surface comprising a single surface conformed along twocurvatures corresponding to two different directions, said two differentdirections corresponding to sagital and meridional directions of theincident X-rays, and said reflecting surface has a sagital curvatureradius of less than 20 mm; and a source of the incident X-rays coupledto the optical assembly so the incident X-rays are conditioned along twodimensions to adapt a beam emitted by the source in destination of asample, with the X-rays having different angles of incidence on thesample.
 40. The device of claim 39, wherein the dispersion of angleincidences on the sample corresponds substantially to an angulardispersion along a sagital dimension of the beam reflected by theoptical assembly.
 41. The device as claimed in one of claims 39 or 40,wherein the optical assembly is directed relative to the sample so thatthe normal in a center region of the optical assembly is approximatelyparallel to the surface of the sample.
 42. The device of claim 39,wherein a capture angle at a level of the sample is greater than 2°along a first dimension corresponding to a sagital dimension of theoptical assembly and about 1° along a second dimension corresponding toa meridional dimension of the optical assembly, the optical assemblybeing positioned so dispersion in angles of incidence of the X-rays onthe sample is greater than 2°, the sample being positioned at least 15cm from the optical assembly.
 43. A device for generating andconditioning X-rays for analytical applications, the device comprising:a X-ray source operable to emit an X-ray beam; and an optical assemblycoupled to the X-ray source, the assembly comprising a laterally gradedreflective multilayer having a reflecting surface to reflect incidentX-rays of the X-ray beam under low incidence angles while producing atwo-dimensional optical effect, the reflecting surface comprising asingle surface conformed along two curvatures corresponding to twodifferent directions; wherein the two different directions correspondrespectively to sagital and meridional directions of the incidentX-rays, and the reflecting surface has a sagital curvature radius ofless than 20 mm; and wherein the optical assembly is operable to focusthe X-ray beam emitted by the X-ray source onto a focalization pointhaving a distance less than 90 cm from the X-ray source.